/*******************************************
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5
******************************************/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* sortedListToBST(ListNode* head) {
      if (!head) {
	return nullptr;
      }

      if (head&&!head->next) {
	return new TreeNode(head->val);
      }

      ListNode* slow = head;
      ListNode* fast = head;
      ListNode* pre = nullptr;
      while (fast&&fast -> next) {
	pre = slow;
	slow = slow -> next;
	fast = fast -> next -> next;
      }

      TreeNode* root = new TreeNode(slow -> val);
      if (pre) {
	pre -> next = nullptr;
      }

      root -> left = sortedListToBST(head);
      root -> right = sortedListToBST(slow -> next);

      return root;
    }
};
